U.S. patent number 7,141,126 [Application Number 10/872,831] was granted by the patent office on 2006-11-28 for rare earth magnet and method for manufacturing the same.
This patent grant is currently assigned to Neomax Co., Ltd.. Invention is credited to Futoshi Kuniyoshi, Hitoshi Morimoto.
United States Patent |
7,141,126 |
Kuniyoshi , et al. |
November 28, 2006 |
Rare earth magnet and method for manufacturing the same
Abstract
Rare earth alloy powder having an oxygen content of 50 to 4000
wt. ppm and a nitrogen content of 150 to 1500 wt. ppm is compacted
by dry pressing to produce a compact. The compact is impregnated
with an oil agent and then sintered. The sintering process includes
a first step of retaining the compact at a temperature of
700.degree. C. to less than 1000.degree. C. for a period of time of
10 to 420 minutes and a second step of permitting proceeding of
sintering at a temperature of 1000.degree. C. to 1200.degree. C.
The average crystal grain size of the rare earth magnet after the
sintering is controlled to be 3 .mu.m to 9 .mu.m.
Inventors: |
Kuniyoshi; Futoshi (Osaka,
JP), Morimoto; Hitoshi (Hyogo, JP) |
Assignee: |
Neomax Co., Ltd.
(JP)
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Family
ID: |
18768011 |
Appl.
No.: |
10/872,831 |
Filed: |
June 22, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040231751 A1 |
Nov 25, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09955306 |
Sep 19, 2001 |
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Foreign Application Priority Data
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Sep 19, 2000 [JP] |
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2000-283680 |
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Current U.S.
Class: |
148/122; 419/33;
419/30; 419/12; 148/104; 419/38; 148/101 |
Current CPC
Class: |
H01F
1/0577 (20130101); H01F 1/0573 (20130101); H01F
41/0266 (20130101); H01F 1/0557 (20130101) |
Current International
Class: |
H01F
1/057 (20060101); H01F 1/08 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
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5489343 |
February 1996 |
Uchida et al. |
5645651 |
July 1997 |
Fujimura et al. |
5788782 |
August 1998 |
Kaneko et al. |
5858123 |
January 1999 |
Uchida et al. |
5968289 |
October 1999 |
Sakurada et al. |
6296720 |
October 2001 |
Yamamoto et al. |
6468365 |
October 2002 |
Uchida et al. |
6482353 |
November 2002 |
Kuniyoshi et al. |
6511631 |
January 2003 |
Kuniyoshi et al. |
6529107 |
March 2003 |
Shimuzu et al. |
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Foreign Patent Documents
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0 753 867 |
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Jan 1997 |
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EP |
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0 994 493 |
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Apr 2000 |
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EP |
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62-170455 |
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Jul 1987 |
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JP |
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63-033505 |
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Feb 1988 |
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JP |
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63-301505 |
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Dec 1988 |
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JP |
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07-018366 |
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Jan 1995 |
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JP |
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08-069908 |
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Mar 1996 |
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JP |
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08-279406 |
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Oct 1996 |
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JP |
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10-012473 |
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Jan 1998 |
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JP |
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10-041174 |
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Feb 1998 |
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JP |
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10-321451 |
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Dec 1998 |
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JP |
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WO 93/20567 |
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Oct 1993 |
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WO |
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Other References
European Search Report (Dated Jul. 24, 2003). cited by other .
Notice of Reasons of Rejection, Patent Application No. 2001-272116,
Mailing Date: Dec. 25, 2001, Mailing No. 593114. cited by other
.
Mikio et al., English language translation of Japanese Patent
Document No. 08-279406, Oct. 22, 1996, Original Japanese document
cited in applicants' IDS submitted Mar. 15, 2002. cited by
other.
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Primary Examiner: Sheehan; John P.
Attorney, Agent or Firm: Nixon Peabody LLP Costellia;
Jeffrey L.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application is continuation of U.S. patent application
Ser. No. 09/955,306, tiled Sep. 19, 2001, now abandoned.
Claims
What is claimed is:
1. A method for manufacturing an R--Fe--B rare earth magnet,
comprising the steps of: preparing a rare earth alloy powder having
an oxygen content in a range of 50 wt. ppm to 4000 wt. ppm and a
nitrogen content in a range of 150 wt. ppm to 1500 wt. ppm and
embrittling an R--Fe--B rare earth alloy by hydrogen occlusion and
milling the embrittled alloy; compacting the rare earth alloy
powder by dry pressing to produce a compact; and, sintering the
compact, wherein the step of sintering the compact includes: a
first step of retaining the compact at a temperature in a range of
700.degree. C. to less than 1000.degree. C. for a period of time in
a range of 10 minutes to 420 minutes and releasing hydrogen outside
the compact so that the amount of hydrogen contained in sintered
magnet is in a range of 10 wt. ppm to 100 wt. ppm; and a second
step of permitting the proceeding of sintering at a temperature in
a range of 1000.degree. C. to 1200.degree. C., and the average
crystal grain size of the rare earth magnet after the sintering is
in a range of 3 .mu.m to 9 .mu.m.
2. A method for manufacturing an R--Fe--B rare earth magnet
according to claim 1, further comprising a step of impregnating the
compact with an oil agent from the surface of the compact, alter
the step of compacting the rare earth alloy powder.
3. A method for manufacturing an R--Fe--B rare earth magnet
according to claim 1, wherein the step of preparing rare earth
alloy powder includes milling an alloy material in a nitrogen gas
atmosphere having an oxygen concentration of 5000 wt. ppm or less
and nitriding the surface of milled powder.
4. A method for manufacturing an R--Fe--B rare earth magnet
according to claim 1, wherein the average particle size of the rare
earth alloy powder is in a range of 1.5 .mu.m to 5.5 .mu.m.
5. A method for manufacturing an R--Fe--B rare earth magnet
according claim 2, wherein the oil agent includes a volatile
component.
6. A method for manufacturing an Re--Fe--B rare earth magnet
according to claim 5, wherein after the step of impregnating the
compact, the temperature of the compact is at least temporarily
reduced due to the volatilization of the oil agent.
7. A method for manufacturing an R--Fe--B rare earth magnet
according to claim 2, wherein the oil agent comprises a hydrocarbon
solvent.
8. A method for manufacturing an R--Fe--B rare earth magnet
according to claim 1, wherein prior to the step of compacting the
rare earth alloy powder, a lubricant is added to the rare earth
alloy powder.
9. A method for manufacturing an R--Fe--B rare earth magnet
according to claim 2, further comprising the step of removing the
oil agent substantially prior to the step of sintering the compact
and alter the step of removing the oil agent, the compact is kept
away from contact with the atmosphere until completion of the step
of sintering the compact.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rare earth magnet and a method
for manufacturing the same. More particularly, the present
invention relates to a high-performance rare earth sintered magnet
manufactured of rare earth alloy powder having a reduced oxygen
content.
2. Description of the Related Art
An R--Fe--B rare earth magnet (R is at least one kind of element
selected from the group consisting of yttrium (Y) and rare earth
elements) is mainly composed of a major phase made of an
R.sub.2Fe.sub.14B tetragonal compound, an R-rich phase including a
rare earth element such as Nd in a large proportion, and a B-rich
phase including boron (B) in a large proportion. The magnetic
properties of an R--Fe--B rare earth magnet are improved by
increasing the proportion of an R.sub.2Fe.sub.14B tetragonal
compound as the major phase in the magnet.
At least a minimum amount of the R-rich phase is necessary for
liquid-phase sintering which is a necessary process for forming
sintered rare earth magnets. Since R reacts with oxygen to generate
an oxide R.sub.2O.sub.3, R is partly consumed prior to the
sintering. Therefore, to compensate for the amount consumed by the
oxidation, an additional amount of R is conventionally required.
The oxide R.sub.2O.sub.3 is generated more vigorously as the amount
of oxygen is greater. In view of this, it has been attempted to
reduce the concentration of oxygen in an atmosphere in which
R--Fe--B alloy powder is produced to suppress generation of the
oxide R.sub.2O.sub.3, and to thereby reduce the relative amount of
the R in the finally manufactured R--Fe--B rare earth magnet and
thus improve the magnetic properties of the magnet.
The amount of oxygen in R--Fe--B alloy powder used for manufacture
of an R--Fe--B magnet should preferably be small, as described
above. However, no attempt to reduce the amount of oxygen in
R--Fe--B alloy powder for improving the magnet properties has been
realized as a mass production technique, for the following reason.
If R--Fe--B alloy powder is produced in a controlled environment of
a low oxygen concentration so that the amount of oxygen in the
alloy powder is as low as 4000 wt. ppm or less, for example, the
powder may vigorously react with oxygen in the atmosphere (the
air), causing the possibility of ignition in several minutes at
room temperature.
Hydrogen processing for milling provides good production efficiency
compared with mechanical milling using a ball mill, for example.
However, when magnet powder produced by the hydrogen processing is
used for manufacture of a magnet, the resultant magnet tends to
vary in magnetic properties (coercive force among others) depending
on the sintering conditions. In particular, the variation in
magnetic properties is significant when the amount of oxygen in the
sintered body is as small as 4000 wt. ppm or less and the total
amount of the rare earth element in the magnet is comparatively
small (e.g., 32 wt. % or less).
Therefore, while it has been recognized that the amount of oxygen
in R--Fe--B alloy powder should desirably be reduced for improving
the magnetic properties, in reality, it is extremely difficult to
handle R--Fe--B alloy powder having a reduced oxygen concentration
in a production site such as a plant.
In particular, the risk of ignition is high during a pressing or
compacting process in which powder is compacted with a press. In
this process, the temperature of a compact rises due to heat
generated as the result of friction among powder particles during
compaction and as a result of friction between powder particles and
the inner sidewall of a cavity of the press during ejection of the
compact. One possible technique for prevention of ignition includes
placement of the press in an environment of a non-oxygen
atmosphere. This placement is however impractical because supply of
the raw material to the press and retrieval of the compact from the
press are difficult in such a non-oxygen environment. The
occurrence of ignition may also be avoided if individual compacts
are immediately sintered when they are ejected from the press. This
is, however, an extremely inefficient process, and thus not
suitable for mass production. A sintering process takes four hours
or more, and it is reasonable that each sintering process is
carried out against a lot of compacts at the same time. In
addition, in mass production facilities, it is difficult to manage
compacts in an environment of an extremely low oxygen concentration
through a series of processing steps from pressing to
sintering.
A liquid lubricant such as fatty ester is often added to fine
powder before the pressing process to improve compressibility or
formability of the powder. By this addition of a liquid lubricant,
thin oily coatings are formed on the surfaces of powder particles.
Such coatings however fail to sufficiently prevent oxidation of the
powder having an oxygen concentration of 4000 wt. ppm or less.
For the above reasons, a slight amount of oxygen is intentionally
introduced into an atmosphere in which an R--Fe--B alloy is milled,
to thereby oxidize thin surfaces of finely milled powder particles
and thus reduce the reactivity of the powder. In an example of such
a technique, Japanese Patent Publication No. 6-6728 discloses a
process in which a rare earth alloy is finely milled under a
supersonic inert gas flow containing a predetermined amount of
oxygen, so that during the milling a thin oxide coating is formed
on the surfaces of fine powder particles produced by the milling.
According to this technique, since oxygen in the atmosphere is
blocked by the oxide coatings on the powder particles, occurrence
of heat generation/ignition due to oxidation is prevented. Note,
however, that with the existence of the oxide coatings on the
surfaces of the powder particles, the amount of oxygen contained in
the powder increases.
U.S. Pat. No. 5,489,343 and Japanese Laid-Open Patent Publication
No. 10-321451 disclose another technique where R--Fe--B alloy
powder having a low oxygen content (for example, 1500 ppm) is mixed
with mineral oil or the like to obtain slurry. Since powder
particles in the slurry are kept from contact with the atmosphere,
occurrence of heat generation/ignition is prevented while the
oxygen content of the R--Fe--B alloy powder is kept low.
This conventional technique has the problem that after the R--Fe--B
alloy powder in the slurry state is filled in a cavity of a press,
the oil must be squeezed out during the pressing process. This
reduces productivity. Further, conventional methods for
manufacturing a rare earth magnet have the problem that crystal
grains tend to become coarse during sintering. The magnet
properties (coercive force) therefore fail to be improved
sufficiently even when magnet powder having a low oxygen
concentration is used.
SUMMARY OF THE INVENTION
A main object of the present invention is providing a
high-performance rare earth magnet having a low oxygen content and
excellent magnet properties, and a method for manufacturing such a
rare earth magnet.
The method for manufacturing an R--Fe--B rare earth magnet of the
present invention includes the steps of: preparing rare earth alloy
powder having an oxygen content in a range of 50 wt. ppm to 4000
wt. ppm and a nitrogen content in a range of 150 wt. ppm to 1500
wt. ppm; compacting the rare earth alloy powder by dry pressing to
produce a compact; impregnating the compact with an oil agent from
the surface of the compact; and sintering the compact. The oil
agent preferably includes a volatile component such as a
hydrocarbon solvent; while, the step of sintering the compact
includes: a first step of retaining the compact at a temperature in
a range of 700.degree. C. to less than 1000.degree. C. for a period
of time in a range of 10 minutes to 420 minutes; and a second step
of continuing the sintering at a temperature in a range of
1000.degree. C. to 1200.degree. C., and the average crystal grain
size of R.sub.2Fe.sub.14B compounds in the rare earth magnet after
the sintering is in a range of 3 .mu.m to 9 .mu.m. The average
crystal grain size of the R.sub.2Fe.sub.14B compounds in the rare
earth magnet after the sintering is more preferably in a range of 3
.mu.m to 6 .mu.m.
Preferably, the method further includes the step of removing the
oil agent substantially prior to the step of sintering the compact,
and after the step of removing the oil agent, the compact is kept
away from contact with the atmosphere until termination of the step
of sintering the compact.
In a preferred embodiment, the step of preparing rare earth alloy
powder includes milling a material alloy in a nitrogen gas
atmosphere having an oxygen concentration of 5000 wt. ppm or less
and nitriding the surface of milled powder. The oxygen
concentration of the nitrogen gas atmosphere is more preferably
2000 wt. ppm or less, and the average particle size (mass median
particle diameter) of the rare earth alloy powder is preferably in
a range of 1.5 .mu.m to 5.5 .mu.m.
In still another preferred embodiment, after the step of
impregnating the compact, the temperature of the compact is at
least temporarily reduced due to volatilization of the oil agent.
Additionally, prior to the step of compacting the rare earth alloy
powder, a lubricant is preferably added to the rare earth alloy
powder.
The R--Fe--B rare earth magnet of the present invention has an
average crystal grain size in a range of 3 .mu.m to 9 .mu.m, an
oxygen concentration in a range of 50 wt. ppm to 4000 wt. ppm, and
a nitrogen concentration in a range of 150 wt. ppm to 1500 wt
ppm.
Alternatively, the method for manufacturing an R--Fe--B rare earth
magnet of the present invention includes the steps of: preparing
rare earth alloy powder having an oxygen content in a range of 50
wt. ppm to 4000 wt. ppm and a nitrogen content in a range of 150
wt. ppm to 1500 wt. ppm by embrittling an R--Fe--B rare earth alloy
by hydrogen occlusion and milling the embrittled alloy; compacting
the rare earth alloy powder to produce a compact; retaining the
compact at a temperature in a range of 700.degree. C. to less than
1000.degree. C. for a period of time in a range of 10 minutes to
420 minutes and releasing hydrogen outside the compact so that the
amount of hydrogen contained in the finally-manufactured magnet is
in a range of 10 wt. ppm to 100 wt. ppm; and sintering the compact
at a temperature in a range of 1000.degree. C. to 1200.degree. C.
The rare earth magnet after the sintering has an average crystal
grain size in a range of 3 .mu.m to 13 .mu.m.
In another alternative, the R--Fe--B rare earth magnet of the
present invention has an oxygen concentration in a range of 50 wt.
ppm to 4000 wt. ppm, a nitrogen concentration in a range of 150 wt.
ppm to 1500 wt. ppm, and a hydrogen content in a range of 10 wt.
ppm to 100 wt. ppm.
In a preferred embodiment, a rare earth element concentration is 32
wt. % or less of the magnet.
The average crystal grain size is preferably in a range of 3 .mu.m
to 13 .mu..mu.m.
The R--Fe--B rare earth magnet is preferably manufactured using an
alloy produced by quenching.
The R--Fe--B rare earth magnet of the present invention has an
oxygen concentration in a range of 50 wt. ppm to 4000 wt. ppm, and
a hydrogen content in a range of 10 wt. ppm to 100 wt. ppm, wherein
a rare earth element concentration is 32 wt. % or less of the
magnet.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view of a press used for
compaction of magnetic powder.
FIG. 2 is a diagram illustrating an impregnation process.
FIG. 3 shows temperature profiles in a sintering process, where 30
denotes a profile in a conventional sintering process and 32
denotes a profile in a sintering process according to the present
invention.
FIG. 4 is graph representation of data shown in Table 2, where the
y-axis represents the coercive force and the x-axis represents the
oxygen content.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, for reducing the oxygen content
of an R--Fe--B rare earth magnet, the concentration of oxygen in
rare earth magnet powder is reduced, and the reactive surfaces of
the magnet powder particles are intentionally nitrided to form a
thin protection film covering the surface of the magnet powder
particles. This addition of nitrogen contributes to suppressing
oxidation of the magnet powder when contact with the
atmosphere.
Further, according to the present invention, sintering is performed
in two stages using a relatively low temperature and a relatively
high temperature. By this two-stage sintering, grain growth during
sintering is suppressed, enabling reduction in the average crystal
grain size of the finally-manufactured sintered magnet.
When it is attempted to mass-produce a sintered magnet using magnet
powder having a low oxygen concentration, a compact of the magnet
powder conventionally tends to cause heat generation/ignition as
described previously. This poses a significant disadvantage to the
mass-production of a sintered magnet. According to the present
invention, in order to solve the problem of heat
generation/ignition of a compact, the surface of magnet powder
particles, having a low oxygen concentration, are nitrided thus
weakening the reactivity of the surface of the particles. In
addition, the resultant powder compact is impregnated with an
organic solvent. An organic solvent contains carbon and other
impurities that are considered improper for rare earth sintered
magnets. However, these impurities are removed sufficiently through
a preheating (oil removing) process prior to sintering and are thus
prevented from adversely influencing the final magnet
properties.
As a result, it is considered that the particle surfaces are not
only suppressed from reacting with oxygen in the atmosphere, but
also suppressed from reacting or binding with the organic solvent.
Thus, carbon and other impurities contained in the organic solvent
can be immediately volatilized/removed from the compact before
sintering. Therefore, deterioration in magnet properties due to the
organic solvent is reliably avoided.
An R--Fe--B rare earth magnet of an embodiment of the invention has
an average crystal grain size in the range of 3 .mu.m to 9 .mu.m,
an oxygen concentration in the range of 50 wt. ppm to 4000 wt. ppm,
and a nitrogen concentration in the range of 150 wt. ppm to 1500
wt. ppm. The "R--Fe--B rare earth magnet" as used herein is defined
to broadly include a rare earth magnet with a metal such as cobalt
(Co) substituting for part of Fe and a rare earth magnet with
carbon (C) substituting for part of boron (B). The R--Fe--B rare
earth magnet has a structure in which R.sub.2Fe.sub.14B compounds
of tetragonal crystals exist as a major phase. The
R.sub.2Fe.sub.14B crystals are surrounded by an R-rich and B-rich
phase (boundary phase) in the R--Fe--B rare earth magnet. The
structure of such an R--Fe--B rare earth magnet is disclosed in
U.S. Pat. No. 5,645,651, which is incorporated herein by
reference.
Hereinafter, a preferred embodiment of the method for manufacturing
such a rare earth magnet will be described in detail.
Initially produced is molten mass of an R--Fe--B alloy containing
about 10 to about 30 at. % of R (at least one kind from the group
consisting of Y and the rare earth elements), 0.5 to 28 at. % of B,
and Fe as the remainder, together with inevitably contained
impurities. Either one or both of Co and Ni may be substituted for
part of Fe, and C may be substituted for part of B. According to
the present invention, the oxygen content can be reduced and thus
production of an oxide of the rare earth element R can be
suppressed. It is therefore possible to keep the amount of the rare
earth element R to its necessary minimum amount.
The molten alloy is then quenched and solidified into a shape of
thin plates having a thickness of 0.03 to 10 mm at a cooling rate
of 10.sup.2 to 10.sup.4.degree. C./sec by a quenching method such
as strip casting, to form cast pieces having a structure with the
R-rich phase having a fine size of 5 .mu.m or less being dispersed.
The cast pieces, accommodated in a case, are placed in a chamber
provided with air intake and outlet facilities. After evacuation of
the chamber, H.sub.2 gas with a pressure of 0.03 to 1.0 MPa
(megapascal) is supplied into the chamber, to form disintegrated
alloy powder. The disintegrated alloy powder is dehydrogenated and
then finely milled under inert gas flow.
The cast pieces as a magnet material used in the present invention
can be produced appropriately by quenching the molten alloy of a
specific composition by strip casting using a single roll method or
a twin roll method. The use of the single roll method or the twin
roll method may be determined depending on the thickness of the
cast pieces to be produced. The twin roll method is preferably used
when thick cast pieces are to be produced, while the single roll
method is preferably used when thin cast pieces are to be produced.
An alloy produced by a quenching method exhibits a sharp particle
size distribution, is uniform in particle size, and thus improves
in the squareness of a demagnetization curve after sintering.
If the thickness of the cast pieces (flake-like alloy) is less than
0.03 mm, the quenching rate is so great that the crystal grain size
may be excessively small. If the crystal grain size is excessively
small, when the cast pieces are powdered the powder particles
individually have a polycrystalline structure. This results in
failure to align the crystal orientation and thus degradation of
the magnetic properties. If the thickness of the cast pieces
exceeds 10 mm, the cooling rate is low. As a result, .alpha.-Fe is
easily precipitates, and also the Nd-rich phase is unevenly
distributed.
The hydrogen processing for embrittlement is performed in the
following manner, for example. The cast pieces crushed to a
predetermined size are put in a material case, and the material
case is placed in a sealable hydrogen furnace, which is then
sealed. After sufficient evacuation of the hydrogen furnace,
hydrogen gas with a pressure of 30 kPa to 1.0 MPa is supplied into
the furnace, to allow the cast pieces to occlude hydrogen. Since
the hydrogen occlusion is exothermic reaction, cooling piping for
flowing cooling water is preferably provided around the furnace to
prevent temperature rise in the furnace. The cast pieces
spontaneously disintegrate due to the hydrogen occlusion and thus
are embrittled (or partially powdered).
The embrittled alloy is cooled and dehydrogenated by heating under
vacuum. The dehydrogenated alloy powder particles have microcracks.
Such particles can be finely milled in a short time during
subsequent milling with a ball mill, a jet mill, or the like. Thus,
alloy powder with a predetermined particle size distribution can be
produced. A preferred embodiment of the hydrogen processing for
milling is disclosed in Japanese Laid-Open Patent Publication No.
7-18366.
The above fine milling is preferably performed with a dry mill such
as a jet mill, an attritor, and a vibration mill, using inert gas
containing nitrogen and containing substantially no oxygen. During
this milling, the oxygen concentration of the inert gas is
preferably controlled at 5000 ppm or less, and a high-purity
nitrogen gas having a purity of 99.99% or more is desirably used as
the inert gas. By milling the powdered alloy in an atmosphere of
such a high-purity nitrogen gas, it is possible to produce finely
milled powder having a low oxygen concentration of which the
particle surfaces have been thinly nitrided. The average particle
size (milled particle size) of the powder is preferably in the
range of 1.5 .mu.m to 5.5 .mu.m, more preferably in the range of
2.5 .mu.m to 5.0 .mu.m.
It is preferable to add to the thus-produced magnet powder a liquid
lubricant containing fatty ester and the like as a major
ingredient. The added amount is 0.15 to 5.0 wt. %, for example.
Examples of the fatty ester include methyl caproate, methyl
caprylate, and methyl laurate. The lubricant may also contain an
ingredient of a binder and the like. Important is that the
lubricant should volatilize and be removed in a subsequent process.
If the lubricant itself is a solid that is not easily mixed with
the alloy powder uniformly, the lubricant may be diluted with a
solvent. As such a solvent, a petroleum solvent represented by
isoparaffin, a naphthenic solvent, and the like may be used. The
lubricant may be added at an arbitrary time, which may be before,
during, or after the milling. The liquid lubricant provides the
effect of protecting the powder particles from being oxidized by
covering the surfaces of the particles. In addition, the liquid
lubricant provides the function of making the green density of a
compact uniform during pressing of the powder and thus suppressing
disorder of alignment.
Next, alignment in a magnetic field (magnetic alignment) and
compaction are performed with a press as shown in FIG. 1. A press
10 in FIG. 1 includes a die 1 having a through-hole and punches 2
and 3 for blocking the through-hole of the die 1 from below and
above. Material powder 4 is filled in a cavity defined by the die
1, the upper punch 3, and the lower punch 2, and compacted by
reducing the gap between the lower punch 2 and the upper punch 3
(pressing process). The press 10 in FIG. 1 also includes coils 5
and 7 for generating an aligning magnetic filed.
The filling density of the powder 4 is set to fall within a range
in which magnetic alignment is possible for the powder. In this
embodiment, the filling density is preferably in the range of 30 to
40% of the true density, for example.
After the powder filling, a magnetic field is applied to the space
filled with the powder 4, to perform magnetic alignment of the
powder 4. This is effective not only for parallel magnetic field
compaction, where the direction of the magnetic field matches with
the pressing direction, but also for vertical magnetic field
compaction where the direction of the magnetic field is vertical to
the pressing direction.
After being ejected from the press 10 in FIG. 1, the compact is
immediately impregnated with an oil agent such as an organic
solvent. FIG. 2 illustrates an impregnation process. In this
embodiment, a solution of saturated hydrocarbon, such as
isoparaffin, is used as the solvent with which a compact 20 is
impregnated. An organic solvent 21 is filled in a bath 22 as shown
in FIG. 2 to allow the compact 20 to be immersed in the organic
solvent 21 in the bath 22. The compact 20 is impregnated or soaked
with the organic solvent 21 from the surface of the compact 20
(i.e., the surface that defines the shape of the compact 20) and
thus substantially covered with the solution of saturated
hydrocarbon. This prevents the compact 20 from being in direct
contact with oxygen in the atmosphere. Therefore, the possibility
of heat generation/ignition of the compact 20 in a short time is
greatly reduced even when the compact 20 is left in the atmosphere.
A half second or longer is enough as the duration of the compact 20
being immersed or soaked in the organic solvent 21 (immersing
time). As the immersing time is longer, the amount of the organic
solvent contained in the compact is larger. A larger amount of the
organic solvent however does not cause a problem such as collapse
of the compact. Therefore, the compact may be kept immersed in the
organic solvent, or the impregnation process may be repeated a
plurality of times, until the sintering process starts.
As the organic solvent used for the impregnation, it is possible to
use a solvent such as the liquid lubricant added to the powder for
improving the formability and the degree of alignment, and the
organic solvent used for diluting the liquid lubricant. The organic
solvent is required to have a function of preventing surface
oxidation. In consideration of this, particularly preferred as the
organic solvent are petroleum solvents represented by isoparaffin,
naphthenic solvents, fatty esters such as methyl caproate, methyl
caprylate, and methyl laurate, higher alcohols, higher fatty acids,
and the like.
After the impregnation, the compact 20 is subjected to known
manufacturing processes including preheating (oil removing),
two-stage sintering, and aging, to be finally completed as a
permanent magnet product. Carbon (C) contained in the oil agent
deteriorates the magnetic properties of the resultant rare earth
magnet. Therefore, as the oil agent with which the compact 20 is
impregnated, must be one that is easily removed from the compact
during preheating and/or sintering is selected. The oil agent is
therefore prevented from adversely influencing the magnet
properties. After volatilization of the oil agent during preheating
before sintering, the compact must be placed in an environment of a
low oxygen concentration to be kept away from contact with the
atmosphere. For this purpose, furnaces for preheating and sintering
are preferably directly coupled with each other so that the compact
can be moved between the furnaces without direct contact with the
atmosphere. A continuous furnace is more desired.
According to the present invention, two-stage sintering is
performed as described above. By the two-stage sintering, it is
possible to control the crystal grain size of the
finally-manufactured sintered magnet in the range of 3 .mu.m to 9
.mu.m, preferably in the range of 3 .mu.m to 6 .mu.m. In the
conventional sintering process, crystal grains become coarse by
grain growth during sintering. For this reason, it is difficult to
improve the coercive force of the magnet sufficiently even when
magnet powder having a low oxygen content is used. According to the
sintering process adopted in the present invention, however, the
effect of using the magnet powder having a low oxygen content can
be sufficiently exhibited.
FIG. 3 shows temperature profiles in the sintering process. In FIG.
3, the reference numeral 30 denotes a profile adopted in the
conventional sintering process, while 32 denotes a profile adopted
in the sintering process according to the present invention.
Two-stage heat treatment is performed in the sintering process in
this embodiment. At the first stage, the compact is kept at a
relatively low temperature (for example, 750 to 950.degree. C.) for
a relatively long period of time (for example, 30 to 360 minutes).
The stage then proceeds to the second stage, where the compact is
kept at a relatively high temperature (for example, 1000 to
1100.degree. C.) for a relatively short period of time (for
example, 30 to 240 minutes).
Hydrogen remaining in the R.sub.2Fe.sub.14B phase as the major
phase during the hydrogen processing for pulverization, which is
the processing utilizing the phenomenon of hydrogen occlusion and
embritlement of the rare earth alloy is released in the preheating
process at about 500.degree. C. performed before the sintering
process. However, the temperature of about 500.degree. C. is not
high enough to dehydrogeneate a rare earth hydrogen compound
(RH.sub.x) formed by the combining between the rare earth element
included in the R-rich phase and hydrogen during the hydrogen
processing for pulverization. In the sintering process according to
the present invention, such a rare earth hydrogen compound
(RH.sub.x) releases hydrogen to form rare earth metal at the first
stage. More specifically, during the first-stage heat treatment at
a temperature of 700.degree. C. or more, there occurs reaction
represented by RH.sub.x.fwdarw.R+(x/2)H.sub.2.uparw.. As a result,
at the second-stage heat treatment, the R-rich phase at the grain
boundary is swiftly turned into a liquid phase, permitting swift
proceeding of the sintering process and shrinkage of the sintered
body. The sintering process is therefore completed in a short
period of time, and this suppresses the crystal grains from
becoming coarse. As a result, the coercive force of the sintered
magnet improves, and also the density of the sintered body
increases.
According to experiments carried out by the present inventors, the
coercive force of a sintered magnet varies with the crystal grain
size of the magnet more significantly when the oxygen content of
the sintered magnet is smaller. For example, when the oxygen
content was 7000 wt. ppm, the difference in coercive force was less
than 10% between a magnet having a crystal grain size of about 3 to
about 6 .mu.m and a magnet having a crystal grain size of about 12
to about 15 .mu.m. When the oxygen content was 3000 wt. ppm, the
difference in coercive force was as large as about 10% or more
between a magnet having an average crystal grain size of 9 .mu.m or
less and a magnet having an average crystal grain size exceeding 9
.mu.m.
In this embodiment, the material alloy was produced by strip
casting. Alternatively, other methods such as ingot casting, direct
reduction, atomizing, and centrifugal casting, may be adopted.
EXAMPLE 1
A molten alloy having a composition of Nd+Pr (30.0 swt. %), Dy (1.0
wt. %), B (1.0 wt. %), and Fe (the balance) was produced in a
high-frequency melting crucible. The molten alloy was then cooled
with a roll-type strip caster to produce thin plate-shaped cast
pieces (flake-like alloy) having a thickness of about 0.5 mm. The
concentration of oxygen contained in the flake-like alloy was 150
wt. ppm.
The flake-like alloy accommodated in a case was then placed in a
hydrogen furnace. After evacuation of the furnace, hydrogen gas was
supplied into the furnace for two hours for hydrogen embrittlement.
The hydrogen partial pressure in the furnace was set at 200 kPa.
After the flakes spontaneously disintegrated due to hydrogen
occlusion, the furnace was evacuated while heating, for
dehydrogenation. Argon gas was then introduced into the furnace,
and the furnace was cooled to room temperature. The alloy was taken
out from the hydrogen furnace when the temperature of the alloy was
lowered to 20.degree. C. At this stage, the oxygen content of the
alloy was 1000 wt. ppm.
The resultant alloy was milled with a jet mill having a milling
chamber filled with a nitrogen gas atmosphere of which the oxygen
concentration was controlled to 200 vol.ppm or less, to produce
magnet powder having various oxygen concentrations. The milling
conditions such as the milling time were adjusted so as to vary the
average particle size (milled particle size) within the range of
1.5 to 7.5 .mu.m, to thereby produce various types of powder having
different average particle sizes. During the milling, also, the
amount of oxygen contained in the nitrogen atmosphere was
controlled so as to vary the oxygen content of the powder with
about 7000 wt. ppm as the maximum. The thus-produced types of
powder had nitrogen concentrations in the range of 100 to 900 wt.
ppm.
Thereafter, 0.5 wt. % of a liquid lubricant was added to the
resultant milled powder with a rocking mixer. As the lubricant, one
containing methyl caproate as a major ingredient was used. Each
type of powder was then compacted by dry pressing with the press
shown in FIG. 1 to produce a compact. The "dry" as used herein is
broadly defined as including the case where the powder contains a
comparatively small amount of a lubricant (oil agent), as in this
example, as long as the process of squeezing the oil agent is not
necessary. The size of the compact was 30 mm.times.50 mm.times.30
mm and the density was 4.2 to 4.4 g/cm.sup.3.
Each compact was then impregnated with an oil agent from the
surfaces thereof. Isoparaffin was used as the oil agent. The
compact was entirely immersed in the oil agent for 10 seconds. The
compact was then taken out from the oil agent, and left standing in
the atmosphere at room temperature. Thereafter, the temperature of
the compact was measured. Heat is generated when a rare earth
element in the compact is oxidized. Therefore, by measuring the
temperature of the compact, the progress of oxidation can be
evaluated.
The temperature of the compact was 40.degree. C. or less
immediately after the impregnation and remained below 50.degree. C.
even after the lapse of 600 seconds. The rise of the temperature of
the compact was terminated after the lapse of about 2000 seconds.
Even the compact produced from the powder having the lowest oxygen
concentration had a maximum temperature of only about 70.degree. C.
Therefore, no possibility of ignition existed even when the compact
was left standing in the atmosphere for a long period of time.
There was observed a phenomenon that the temperature of the compact
reduced temporarily (a few minutes) after the impregnation. This is
because the oil agent volatilized from the surface of the compact
and the compact was cooled due to heat of vaporization.
The case of performing no impregnation with an oil agent for a
compact (comparative example) was also examined. A compact of which
the oxygen concentration was adjusted to about 2000 wt. ppm or less
ignited in the atmosphere about two minutes after ejection from the
press. A compact of which the oxygen concentration was about 3000
wt. ppm continued temperature rise from immediately after pressing
and reached as high as 90.degree. C. before the lapse of 600
seconds, causing the risk of ignition. Heat generated by oxidation
facilitates oxidation of surrounding powder. Therefore, once
oxidation starts, the temperature of the compact sharply rises, and
the risk of ignition significantly increases. Such a compact
presumably continues being gradually oxidized and accumulates heat
inside even when the compact is placed in a container filled with
an atmosphere having a comparatively low oxygen concentration.
Therefore, the compact will sooner or later generate heat sharply,
causing the risk of ending up with ignition.
The compacts coated with the oil agent were preheated at
250.degree. C. for two hours for oil removal, and then sintered
under the conditions shown in Table 1 below. Table 1 shows the
particle size of powder before sintering (milled particle size) and
the average crystal grain size after sintering. The milled particle
size is a median size measured with a He--Ne laser diffraction-type
particle size distribution measuring apparatus (for example, HELOS
& RODOS type available from Sympatec Corp.), and the average
crystal grain size of the R.sub.2Fe.sub.14B phase was measured
according to a cutting method defined by JIS H 0501.
TABLE-US-00001 TABLE 1 Sample No. 1 2 3 4 Milled particle 1.5 3.5
3.5 5.5 3.5 5.5 5.5 7.5 size (.mu.m) Sintering 800.degree. C.
800.degree. C. 1060.degree. C. 6 hrs 1060.degree. C. 6 hrs.
Conditions 4 hrs. + 4 hrs. + 1050.degree. C. 1050.degree. C. 2 hrs.
2 hrs. Crystal grain 3 6 6 9 9 12 12 15 size (.mu.m)
Various magnetic properties were measured for the sintered magnets
manufactured under the above conditions. Table 2 below shows how
the magnetic properties change depending on the oxygen
concentration of powder used for compaction.
TABLE-US-00002 TABLE 2 Sample No. 1 2 3 4 Oxygen Coercive Coercive
Coercive Coercive content force force force force (wt. ppm) (kA/m)
(kA/m) (kA/m) (kA/m) 1200 1230 1200 1080 900 2000 1200 1180 1050
890 2500 1200 1110 1000 850 3100 1130 1080 1000 860 4200 1000 1020
1000 840 5500 820 780 780 750 7000 600 580 570 580
FIG. 4 is a graph prepared based on the data shown in Table 2. The
y-axis and the x-axis of this graph respectively represent the
coercive force (kA/m) and the oxygen content (wt. ppm). The oxygen
content, which indicates the concentration of oxygen contained in
the magnet after the sintering, was measured by a non-dispersion
infrared detection method. The nitrogen content was measured by a
thermal conductivity detection method. Specifically, the oxygen
content and the nitrogen content were measured with a measuring
apparatus (EMGA-550) available from Horiba, Ltd.
As is apparent from Table 2 and FIG. 4, the coercive force is
higher as the crystal grain size after sintering is smaller and the
oxygen concentration is lower. When the oxygen concentration after
the sintering process is high (for example, 7000 wt. ppm), the
coercive force is low irrespective of the crystal grain size. On
the contrary, when the oxygen concentration is low, the coercive
force clearly depends on the crystal grain size.
It was also found that although the milled particle size was in the
range of 3.5 to 5.5 .mu.m, the crystal grains became coarse when
the two-stage sintering was not performed. In this case, therefore,
the effect of providing a high coercive force by reducing the
oxygen concentration was not sufficiently exhibited.
In consideration of the above, the crystal grain size should
preferably be made small by adopting the two-stage sintering
process when, in particular, a sintered magnet is to be
manufactured using magnet powder having a low oxygen concentration.
For example, when the oxygen concentration of the sintered magnet
is in the range of 1000 wt. ppm to 4000 wt. ppm, the average
crystal grain size of the sintered magnet should preferably be in
the range of 3 .mu.m to 9 .mu.m.
The case of performing the fine milling in an atmosphere of helium
(He) and argon (Ar), for example, was also examined. In this case,
the surfaces of powder particles were not nitrided. Since no
nitride layers were formed on the surfaces of the powder particles,
the powder was easily oxidized causing ignition during the process
and deterioration of the magnetic properties. On the contrary, when
the surfaces of the powder particles were nitrided excessively, the
sintering process proceeded less smoothly, resulting in
deterioration of the magnetic properties. In view of these, the
nitrogen concentration in the magnet powder should preferably be
controlled in the range of 150 wt. ppm to 1500 wt. ppm, more
preferably in the range of 200 wt. ppm to 700 wt. ppm.
The method for impregnating the surface portion of a compact with
an oil agent is not limited to that described above. A spraying
method, a brushing method, or the like may also be adopted, and in
such a case, substantially the same effect can be obtained.
The composition of the material for the rare earth magnet used in
the present invention is not limited to that described above. The
present invention is broadly applicable to any types of rare earth
alloy powder having a low oxygen concentration that have the risk
of heat generation/ignition due to oxidation in the atmosphere.
A second embodiment of the present invention will be described. As
described in the first embodiment, an R--Fe--B rare earth magnet of
which the oxygen content has been reduced to enhance the
performance can exhibit an increased residual flux density Br while
maintaining a high coercive force. In the first embodiment,
however, the magnet properties may deteriorate (in particular, the
coercive force may decrease) and a sufficient density may not be
secured depending on the sintering conditions. This problem is
serious when the content of the rare earth elements in the magnet
is small, for example 32 wt. % or less, particlarly 31 wt. % or
less. To conduct mass-production of the rare earth magnet, the rare
earth element concentration in the magnet is preferably 29 wt. or
more in view of remanence Br and coercive force H.sub.cJ, the rare
earth element concentration is more preferably in the range of 29.5
wt. % to 31 wt. %. Therefore the above problem should be resolved.
The present inventors closely examined this problem, and found that
the occluded hydrogen might not be released sufficiently by the
heat treatment at a temperature in the range of 700.degree. C. to
less than 1000.degree. C. (first stage of the two-stage sintering)
depending on the temperature and the duration of the heat
treatment. In such a case, hydrogen remains in the compact and
causes variation or deterioration of the magnet properties. This is
considered to occur because the compact starts shrinking from the
outer portion thereof during sintering and thus hydrogen gas inside
the compact finds difficulty in coming out for release.
In this embodiment, to attain a high coercive force with good
reproducibility, a sufficiently large amount of hydrogen is
released from the compact at the first stage of the two-stage
sintering in order to control the amount of hydrogen contained in
the finally-manufactured magnet to 100 wt. ppm or less. By this
control, a sintered magnet with excellent magnet properties can be
stably provided.
The thus-manufactured R--Fe--B rare earth magnet of this embodiment
has a hydrogen content controlled to be in the range of 10 wt. ppm
to 100 wt. ppm, in addition to an oxygen concentration in the range
of 50 wt. ppm to 4000 wt. ppm and a nitrogen concentration in the
range of 150 wt. ppm to 1500 wt. ppm. The hydrogen content is
preferably as small as possible. However, if the heat treatment in
the range of 700.degree. C. to less than 1000.degree. C. continues
for a long time to release hydrogen from the compact, grain growth
proceeds, though slowly. This is the reason why the lower limit of
the hydrogen content is set at 10 wt. ppm. From the standpoint of
attaining excellent magnetic properties, the hydrogen content is
more preferably 80 wt. ppm or less.
To manufacture a magnet using powder produced by hydrogen
processing for pulverization while controlling the hydrogen content
of the magnet to be within the above range, attention must be paid
to the conditions at the first stage of the two-stage sintering.
The first-stage sintering is performed at a temperature in the
range of 700.degree. C. to less than 1000.degree. C. If the
temperature and the time of the heat treatment are combined
improperly, the amount of hydrogen contained in the sintered magnet
falls outside the above range. Hydrogen is released from the
compact most effectively at a temperature in the range of
800.degree. C. to 950.degree. C. Therefore, the hydrogen release
amount can be changed by retaining the temperature at 900.degree.
C., for example, and varying the retaining time appropriately. When
the temperature is retained at 900.degree. C. at the first stage,
the retaining time is preferably controlled to 30 minutes or more
to secure the hydrogen content of 100 wt. ppm or less.
The average crystal grain size is preferably controlled to be in
the range of 3 .mu.m to 13 .mu.m, more preferably in the range of 3
.mu.m to 9 .mu.m, to attain a high coercive force.
Hereinafter, an example of the magnet of this embodiment will be
described.
EXAMPLE 2
As in Example 1, a molten alloy having a composition of Nd+Pr (30.0
wt. %), Dy (1.0 wt. %), B (1.0 wt. %), and Fe (the balance) was
produced in a high-frequency melting crucible. The molten alloy was
then cooled with a roll-type strip caster to produce thin
plate-shaped cast pieces (flake-like alloy) having a thickness of
about 0.5 mm. The concentration of oxygen contained in the
flake-like alloy was 150 wt. ppm.
The flake-like alloy accommodated in a case was then placed in a
hydrogen furnace. After evacuation of the furnace, hydrogen gas was
supplied into the furnace for two hours for hydrogen embrittlement.
The hydrogen partial pressure in the furnace was set at 200 kPa.
After the flakes spontaneously disintegrated due to hydrogen
occlusion, the furnace was evacuated while heating for
dehydrogenation. Argon gas was then introduced into the furnace,
and the furnace was cooled to room temperature. The alloy was taken
out from the hydrogen furnace when the temperature of the alloy was
lowered to 20.degree. C. At this stage, the oxygen content of the
alloy was 1000 wt. ppm.
The resultant alloy was milled with a jet mill having a milling
chamber filled with a nitrogen gas atmosphere of which the oxygen
concentration was controlled to 200 wt. ppm or less, to produce
magnet powder having an average particle size (milled particle
size) in the range of 3.5 .mu.m to 5.5 .mu.m. During the milling,
also, the oxygen amount contained in the nitrogen atmosphere was
controlled so that the oxygen content of the powder was in the
range of 2200 to 2300 wt. ppm. The resultant powder had a nitrogen
concentration in the range of 200 to 400 wt. ppm.
Thereafter, 0.5 wt. % of a liquid lubricant was added to the milled
powder with a rocking mixer. As the lubricant, one containing
methyl caproate as a major ingredient was used. The powder was then
compacted by die pressing method in an aligning magnetic field of
0.8 MA/m, to produce a compact. The size of the compact was 30
mm.times.50 mm.times.30 mm and the density was 4.2 to 4.4
g/cm.sup.3.
As in Example 1, the compact was then impregnated with an oil agent
from the surfaces thereof. Thereafter, the compact was subjected to
two-hour preheating at 250.degree. C. for oil removal, and then
sintered under the conditions shown in Table 3 below.
TABLE-US-00003 TABLE 3 Sample No. 5 6 7 8 9 Milled 3.5 5.5 3.5 5.5
3.5 5.5 3.5 5.5 5.5 7.5 particle size (.mu.m) Sintering 900.degree.
C. 900.degree. C. 900.degree. C. 1050.degree. C. 1070.degree. C.
conditions 3 hrs. + 1 hr. + 5 hrs. + 4 hrs. 4 hrs. 1050.degree. C.
1050.degree. C. 1050.degree. C. 4 hrs. 4 hrs. 6 hrs. Crystal grain
8 10 8 10 10 13 7 9 14 18 size (.mu.m)
Sintering under the conditions shown in Table 3 was performed for
each sample in a decompressed Ar gas atmosphere of about 2.5 kPa.
The peak temperature at which a rare earth hydrogen compound
releases hydrogen is in the range of 800.degree. C. to 900.degree.
C. The samples of sintered magnets manufactured under the above
conditions were measured for the oxygen amount, the nitrogen
amount, the hydrogen amount, the sintering density, and the
magnetic properties, and the results are shown in Table 4
below.
TABLE-US-00004 TABLE 4 Sample No. 5 6 7 8 9 Oxygen amount 2500 2500
2600 2700 2600 (wt. ppm) Nitrogen amount 280 290 290 280 280 (wt.
ppm) Hydrogen 40 85 100 120 115 amount (wt. ppm) Sintered body 7.55
7.55 7.50 7.44 7.45 density (g/cm.sup.3) Coercive force 1200 1120
1010 820 740 iHc (kA/m)
As is found from Table 4, while the hydrogen amount was controlled
to fall within the range of 10 to 100 wt. ppm in samples 5 to 7, it
exceeded 100 wt. ppm in the other samples. To set the hydrogen
amount in the range of 10 to 100 wt. ppm, good coercive force can
be obtained. To increase coercive force of the magnet, it is
preferably that the hydrogen amount in the magnet is set to be 85
wt. ppm or less. In samples 8 and 9, where the sintering was
performed only at 1050.degree. C. or more, omitting the stage of
retaining the compact at a temperature in the range of 800.degree.
C. to 900.degree. C., it is considered that part of hydrogen
contained in the surface portion of the compact was released from
the compact in the course of temperature rise.
Thus, in this embodiment, a rare earth hydrogen compound (RH.sub.x)
contained in the grain boundary phase can be sufficiently
dehydrogenated prior to start of the second-stage sintering (prior
to change of the grain boundary phase into the liquid phase). This
improves the sintering density and provides excellent magnet
properties. The resultant magnet according to the present invention
has a low hydrogen concentration compared with the conventional
magnet.
In the above embodiments, dry pressing was adopted. Alternatively,
wet pressing as disclosed in U.S. Pat. No. 5,489,343 may be
adopted. Since the effect of the present invention obtained by
reducing the hydrogen concentration is provided irrespective of the
type of the pressing method, the magnetic properties also improve.
In addition, in the case of adopting wet pressing to produce a
compact, the process of impregnating the compact with an oil agent
after the pressing may be omitted.
Thus, according to the present invention, the sintering process is
performed in two stages of using a relatively low temperature and
using a relatively high temperature. By this two-stage sintering
process, crystal grains are suppressed from becoming coarse, and
the hydrogen content is reduced. As a result, the effect of
increasing the coercive force by reducing the oxygen concentration
can be exhibited satisfactorily. In addition, according to the
present invention, since the compact is impregnated with an oil
agent from the surface thereof, oxidation of the powder compact is
suppressed while the oxygen content of the magnet powder is
reduced. Therefore, the risk of heat generation/ignition can be
reduced, and this makes it possible to safely and practically
increase the amount of the major phase of the magnet. As a result,
the magnet properties of the rare earth-magnet are greatly
improved.
Moreover, according to the present invention, the surfaces of the
material powder particles are properly nitrided. Therefore, the
surfaces of the powder particles are prevented from oxidation
although the oxygen content of the magnet powder is small. As a
result, the amount of the major phase of the magnet increases, and
thus the magnet properties are improved.
Although nitrogen is used as an inert gas for milling process in
the above embodiments argon and/or helium can be used instead of
nitrogen or in addition to nitrogen for milling process.
While the present invention has been described in a preferred
embodiment, it will be apparent to those skilled in the art that
the disclosed invention may be modified in numerous ways and may
assume many embodiments other than that specifically set out and
described above. Accordingly, it is intended by the appended claims
to cover all modifications of the invention that fall within the
true spirit and scope of the invention.
* * * * *